There is provided a fuse block 3 having an insertion port 7 into which a dark current fuse 11 is inserted and adapted to hold the dark current fuse 11 in a mount position, and the fuse block includes a guide surface 8 which is formed within the insertion port 7 in such a manner as to continue from the mount position for the dark current fuse 11 for contact with the dark current fuse 11 and arms formed at positions lying at lateral sides of the dark current fuse 11 in such a manner as to face the insertion port 7 so as to be able to hold the dark current fuse 11 therein when the dark current fuse 11 is in contact with the guide surface 8.